The present invention relates to a low power audio device, such as a low power radio. Such a low power device can dramatically reduce battery consumption, can reduce the role of a battery to that of a secondary storage device, or make the batteries redundant in favour of a solar or mechanical energy source.
Radio remains a powerful communications medium in many parts of the world. Conventional radios are relatively power hungry devices. This is not a problem in affluent societies, but in more remote societies the cost of batteries may make them prohibitively expensive to the local population, thereby causing radios to be under-utilised. Thus, the potential to broadcast messages concerning public health and safety is diminished since many of the intended audience may not be listening.
According to the present invention, there is provided an electrical device having an audio output, the device comprising a signal processing circuit, a class D amplifier, and an audio output element, the amplifier and the signal processing circuit operating at respective supply voltages.
It is thus possible to tailor the supply voltages of the individual parts of the electrical device so that they operate on the minimum supply voltage consistent with their function.
Preferably, the class D amplifier is arranged to produce the supply for the signal processing circuit.
The class D amplifier may be configured to have a single ended output stage. Such an arrangement allows the voltage swing across the output element to be substantially equal to the supply voltage of the class D amplifier.
Preferably, the class D amplifier is configured to have a xe2x80x9cHxe2x80x9d bridge output stage. This gives an enhanced peak-to-peak voltage swing across the audio output element. The peak-to-peak voltage is substantially equal to twice the supply voltage of the class D amplifier.
Preferably, the audio output is a loudspeaker. The loudspeaker may be a moving coil loudspeaker. However, for improved efficiency, especially at higher volumes, a piezoelectric transducer may be used.
Preferably, the class D amplifier is placed in a shielded enclosure with the leads into and out of the enclosure being filtered to reduce electromagnetic interference.
Advantageously, a clock generator and comparator for producing a pulse width modulated drive to the class D amplifier is also contained within the enclosure. Advantageously, the switching clock frequency is in the range of 25 kHz to 120 kHz.
Preferably, the switching frequency is 90 kHz when the output element is a piezoelectric transducer. Such a frequency represents a compromise between reducing ripple current to the transducer (ripple current reducing with higher switching frequency) and keeping switching losses low (switching losses increasing with increasing frequency). A lower switching frequency can be used to drive a moving coil loudspeaker.
Preferably, the clock generator for use in the pulse-width modulation scheme is used to drive a flip-flop configured to produce a square wave output at half the clock frequency. The average voltage of the square wave is half the flip-flop supply voltage. Thus, the output signal of the flip-flop can be low pass filtered to derive a direct current supply of substantially 1.5 volts when the flip-flop is driven from a 3 volt supply. This reduced supply voltage can be used to power the signal processing circuit. Alternatively, a comparator may be used to compare the voltage supplied to the processing circuit with a reference voltage. This circuit can be arranged to cause a charge on a storage capacitor to be topped up via a semiconductor element used in a switching mode when the voltage falls below a preset threshold.
Preferably, the signal processing circuit is a radio receiver.
Advantageously, the receiver will be a superheterodyne design.
Preferably, at least the resonant circuits in the intermediate frequency amplifier-section of the superheterodyne receive will be formed from inductor capacitor combinations. The use of LC resonant circuits was commonplace in the early days of radio, but recently has given way to the use of resistive loads especially in the case of integrated circuits, with ceramic filters being used between stages to give the arrangement the required selectivity. Resistive loads are relatively inefficient and broadband. Ceramic filters give a narrow-band response but are attenuating devices. The use of an LC resonant circuit provides a tuned load with a high impedance at resonance and consequently allows the required amount of gain to be achieved with fewer amplification stages, and with stages operating on a lower quiescent current than is normally chosen. Thus, the power supply requirements of the radio receiver can be reduced thereby enabling further gains in battery life to be achieved.
Preferably, the signal processing circuit is implemented, at least in part, within an integrated circuit and the transistor circuits within the circuit are optimised to achieve high gain-bandwidth products at low quiescent current. The integrated circuit may incorporate connection to external tuned loads, thereby allowing both the advantages of IC fabrication and efficient loads to be enjoyed.
Advantageously, the radio will provide at least one of AM reception in the 500 kHz to 1600 kHz range, FM reception in the 88 MHz to 108 MHz range and shortwave reception on any one or more of the shortwave bands. The FM receiver circuit may utilise a conventional 10.7 MHz intermediate frequency and quadrature detector for signal discrimination. The AM receiver section may use the conventional 465 kHz intermediate frequency.